The invention relates to an arrangement for transmission of a predetermined frequency by means of a tunable high frequency transmitter tube, such as a magnetron comprising a rotatably journalled tuning body and a stator part. The body, by its rotation in a given first direction, produces a periodically repeated continuous variation of the tuning frequency of the tube. The arrangement comprises a mechanical locking device between the rotatable tuning body and the stator part, which device is inoperative in case of rotation in the first direction, but which operates upon rotation in the opposite direction to lock the the tuning body in an angular position relative to the stator part, which position corresponds to the predetermined fixed frequency.
An arrangement of this kind is described in Swedish Pat. No. 349303. The locking device here consists of a locking hook swingably journalled on the stator part and adapted to cooperate with a locking disc connected to the tuning body and having a locking notch, into which the locking hook can fall for locking the disc and thereby the tuning body upon rotation of the body in the operative direction of the locking device. For switching the locking hook between operative and inoperative position, the hook is frictionally controlled from the rotating body by means of one or more frictional springs bearing against the locking disc, so that the locking hook upon rotation of the body in one direction is swung to the inoperative position and upon rotation in the opposite direction is swung towards the locking disc for engaging the locking notch. The locking hook is thereafter maintained in engagement in the locking notch by continuously applying torque to the tuning body in the last direction.
A drawback of this known arrangement is that for its switching function it requires continuous friction between the springs and the locking disc. This appears to result in a rapid wear-out of the springs, i.a. metallic particles produced by the wear can penetrate into the journal bearings of the rotation body, whereby the life length of the tube is heavily reduced.
In order to avoid this a construction was developed and used, which construction had the same principle shape as the described one but in which the frictional springs were replaced by a spring influencing the locking hook for steadily urging the hook towards the locking disc with the locking notch. In normal operation, when the tuning body rotates with high speed, the locking hook then for each revolution will slip over the cam surface preceeding the locking notch on the locking disc and in a snapwise manner fall into the locking notch. Also in this case there will be a certain amount of wear of the cam surface and the stop surface of the locking notch. This construction has proved to give a life length corresponding to approximately 5000 reversing actions in normal use of the tube. A drawback of this construction is that it is not free of maintenance as it requires lubrication.